Preparation of metal dialkyl dithiophosphates



United States Patent 01 ice 3,426,054 PREPARATION OF METAL DIALKYLDITHIOPHOSPHATES Helmuth G. Schneider, Westfield, and Thomas P. Mc-

Namara, Atlantic Highlands, N.J., assignors to Esso Research andEngineering Company, a corporation of Delaware No Drawing. Filed July16, 1964, Ser. No. 383,219 U.S. Cl. 260--429.9 9 Claims Int. Cl. C07f9/16, 3/07 ABSTRACT OF THE DISCLOSURE In preparing a polyvalent metalsalt of a dialkyl dithiophosphoric acid having alkyl groups of from 1 to30 carbon atoms, an alkali metal salt is first prepared and the latteris then converted to a polyvalent metal salt by double decomposition.The contribution of the present invention to the art consists inimproving the quality of the products by keeping the proportion ofdialkyl dithiophosphoric acid to alkali metal base within the narrowlimits of one mole of the former to from 1.1 to 1.15 moles of the latterand by also keeping the proportion of polyvalent metal salt to dialkyldithiophosphoric acid within very narrow limits in the doubledecomposition step, these limits being from 0.5 to 0.55 mole of divalentmetal salt, or from 0.35 to 0.40 mole of trivalent metal salt, per moleof dialkyl dithiophosphoric acid.

The present invention is concerned with an improved process for thepreparation of polyvalent metal salts of dialkyl dithiophosphoric acids.The invention is particularly directed to the preparation of the zincsalts of dialkyl dithiophosphoric acids for use as additives inlubricating oil compositions wherein they serve as antioxidants andwear-reducing agents.

Polyvalent metal salts of dialkyl dithiophosphoric acids whose alkylgroups contain in the range of from about 1 to about 30 carbon atoms andmore particularly those having from about 3 to about 12 carbon atoms arewell known in the art as additives for lubricating oil compositions.Metal salts of this type, and especially the zin salts, are particularlyvaluable as antiwear and antioxidant additives for lubricating oils thatare intended for use in the crankcases of internal combustion engines.The nickel salts have been similarly employed, as have those of cadmiumand lead. Other polyvalent metal salts of dialkyl dithiophosphoric acidshave been found to have antioxidant, corrosion inhibiting or detergentproperties in lubricants, fuels, and related oleaginous compositions.

It is common practice to prepare dialkyl dithiophosphoric acids byreacting phosphorus pentasulfide with an aliphatic alcohol or a mixtureof alcohols containing the desired range of alkyl groups in a molarratio of approximately 4 moles of alcohol for each mole of phosphoruspentasulfide. The acids are then ordinarily neutralized in accordancewith prior art processes With an oxide, hydroxide or carbonate of apolyvalent metal or alternatively with a reactive polyvalent metal salt.

In the processes of the prior art, certain disadvantages have beenencountered in preparing polyvalent metal dialkyl dithiophosphates,particularly when neutralizing the dithiophosphoric acids with metaloxides. For example,

utilization of the metal oxide to obtain the desired degree ofneutralization of acid has been very erratic both as to the proportionof metal oxide required relative to the acid that is being neutralizedand as to the reaction time that is needed to bring about completeneutralization. In the case of zinc oxide neutralization of dialkyldithiophosphoric acids, for example, it has been necessary to 3,426,054Patented Feb. 4, 1969 employ from 5 to 20% excess of zinc oxide and thisin turn increases the time that is required for the final filtrationstep to remove excess oxide.

In accordance with the present invention it has been found that theconversion of dialkyl dithiophosphoric acids to their polyvalent metalsalts can be much more closely controlled and the preparation of thosesalts can be greatly accelerated if a double-decomposition method isemployed rather than the direct neutralization procedure. While it isknown in the prior art to prepare polyvalent metal salts of dialkyldithiophosphoric acids by neutralizing those acids with an alkali metaloxide or hydroxide and then converting the salts to polyvalent metalsalts by a double-decomposition reaction, the process of the presentinvention differs from the prior art methods in that the proportions ofdialkyl dithiophosphoric acid to alkali metal oxide or hydroxide and theproportions of polyvalent metal to dialkyl dithiophosphoric acid arekept within very narrow limits.

The principles of this invention are applicable not only to thepreparation of zinc salts but also to the preparation of salts of othermetals of Group II of the Periodic Table, such as mercury, cadmium,calcium, and barium, as well as amphoteric metals that will form basicsalts, including aluminum, tin, copper, lead, cobalt, nickel, iron,bismuth, and beryllium.

To prepare polyvalent metal salts of dialkyl dithiophosphoric acids inaccordance with the present invention, those acids are first convertedto alkali metal salts by neutralizing the salts with an alkali metalhydroxide in which the mole ratio of dialkyl dithiophosphoric acid toalkali metal hydroxide is kept within the narrow range of 1 mole of acidto from 1.10 to 1.15 moles of alkali metal hydroxide. The resultingalkali metal salts are converted to polyvalent metal salts by adouble-decomposition reaction in which the amount of polyvalent metalsalt employed in the double-decomposition reaction is in the range offrom 0.5 to 0.550 mole of inorganic salt of a. divalent metal, or from0.35 to 0.40 mole of inorganic salt of a trivalent metal, per mole ofdialkyl dithiophosphoric acid.

If the neutralization of dialkyl dithiophosphoric acids and thesubsequent conversion of the alkali metal salt to polyvalent metal saltsare conducted with stoichiometric proportions of alkali metal hydroxideand of polyvalent metal inorganic salt, the products will not besatisfactory in that they will have low pH values, will tend to evolvehydrogen sulfide on storage, and will also tend to be corrosive towardcopper. In the case of zinc salts of mixed C -C dialkyl dithiophosphoricacids, for example, it has been determined that the product made withstiochiometric proportions of reactants has a pH of about 4.0 to 4.5,whereas, prior experience has indicated that it is desirable to have aminimum pH value in the range of about 5.65 to 5.8 in order to obtain aproduct that is stable with respect to H 8 evolution. To obtain thedesired minimum pH value, it is necessary to impart a slight degree ofalkalinity to the product. At the same time, if the excess alkalinity istoo great, the yield falls oif rapidly. The dialkyl dithiophosphoricacids that are employed in the practice of this invention may beobtained from any suitable source. The usual method for preparing theseacids is to react about 4 moles of an aliphatic alcohol or a mixture ofalcohols with 1 mole of phosphorus pentasulfide at a reactiontemperature in the range of about to 250 F. for in the range of fromabout 1 to 6 hours. However, the present invention is not limited by themanner in which the dialkyl dithiophosphoric acids are prepared.

The dialkyl dithiophosphoric acids used in this invention include notonly those made from a simple aliphatic alcohol such as isopropyl,n-butyl, n-decyl, etc., but also mixed aliphatic alcohols such as C C orC oxo alcohols obtained by a reaction of olefins with carbon monoxideand hydrogen and subsequent hydrogenation of the resultant aldehydes.Mixed alcohols obtained by the hydrogenation of natural fats and oilsmay also be employed. For example, mixed aliphatic alcohols in the (I -Crange consisting chiefly of lauryl alcohol can be obtained byhydrogenating cocoanut oil. These are sold under the trade name Lorol.Mixed C C alcohols consisting principally of C and C aliphatic alcoholscan be obtained from tallow by hydrogenation and/or by sodium reduction. Primary aliphatic alcohols of 22 carbon atoms or more can beobtained by the hydrolysis of Ziegler-type ethylene polymers and areavailable commercially from the Continental Oil Company under the nameof Alfol alcohols. All of these higher alcohols can be used for dialkyldithiophosphoric acid manufacture. Dithiophosphoric acid-s obtained frommixtures of alcohols are particularly preferred. Such mixtures include,for example, a combination of isopropyl alcohol and methyl isobutylcarbinol; a combination of primary amyl alcohol and isobutanol; acombination of mixed amyl alcohols and technical lauryl alcohol; amixture of isopropyl alcohol and C oxo alcohol, and the like. It is alsowithin the contemplation of the invention to employ in the preparationof the metal salts mixed acids obtained by reacting individual alcoholsseparately with P 8 The products of reaction of phosphorus pentasulfidewith aliphatic alcohols do not constitute entirely of dialkyldithiophosphoric acids but, in addition, contain hydrogen sulfide andalso materials which have been referred to variously as inerts orneutrals because they contain unacidified sulfur. It is believed thatthese materials are primarily sulfides, disulfides or trisulfides of thedialkyl dithiophosphoric acids. To the extent that hydrogen sulfide ispresent in the dialkyl dithiophosphoric acids, it consumes alkali metalhydroxides in the neutralization step. The alkali metal sulfides orhydrosulfides thus formed react with the polyvalent metal salt in thesubsequent double-decomposition step to form the correspondingpolyvalent metal sulfide. It is thus evident that the presence ofhydrogen sulfide needlessly runs up chemical composition in the process.For this reason it is advantageous to blow the dialkyl dithiophosphoricacids with air or with an inert gas until they are essentially H S freebefore undertaking the neutralization step.

As has been already stated, the dialkyl dithiophosphoric acids alsocontain inerts. To determine the amount of alkali metal hydroxide thatwill be needed for proper neutralization in accordance with the presentinvention, it is first necessary to determine the percentage of freedialkyl dithiophosphoric acids that are present, which is done bytitrating small samples. Then based on the results of this titration,the quantity of alkali metal hydroxide needed to react with the dialkyldithiophosphoric acid in the desired 1.1 to 1.15/1 molar proportion canbe readily determined. The neutralization of the dialkyldithiophosphoric acids is an ionic reaction and, therefore, occurs quiterapidly; however, hydrolysis of the neutrals or inerts is much slowerand requires about 15 minutes. The time needed for completeneutralization for any dialkyl dithiophosphoric acid system can bedetermined by noting the change in free alkalinity with time, which canbe done conveniently by titration of a sample. When no further change inalkalinity occurs, hydrolysis is complete.

The neutralization step can be conducted at any suitable temperature,ranging from ambient temperatures to about 180 F., and it can be carriedout in any suitable vessel, including mixing columns and storage tanks.External cooling will be desirable because the heat of reaction -willraise the temperature. It is advantageous to keep the neutralizationtemperature as low as practical, consistent with the rate of hydrolysisof the inerts because there is a tendency for the filterability of thefinal metal dithiophosphate to decrease with increase in neutralizationtemperature. For mixed C -C dialkyl dithiophosphoric acids, thepreferred temperature. range is about to F.

When neutralizing the dialkyl dithiophosphoric acids with sodiumhydroxide, it is generally advantageous to use as high a strength ofNaOH as possible, the only limitation being that the sodium salt that isformed should not set up to a gel. Gel formation depends on the time ofaging and the type of alcohols that have been used. For alcohols of 5carbon atoms, it has been found that the sodium hydroxide concentrationcan be as high as 19 to 20 weight percent. With higher molecular weightdialkyl dithiophosphoric acids, there is a greater tendency toward gelformation upon neutralization with NaOH. For example, if the acids havebeen derived from hexylalcohols, it is desirable to lower the NaOHconcentration to 18 weight percent. Gel formation per se does not affectthe quality of the final additive formed on double decomposition; itdoes, nevertheless, make the handling of the alkali metal salt of thedialkyl dithiophosphoric acids difiicult. Gel formation also has atendency to render the polyvalent metal salt of the dialkyldithiophosphoric acids of such small particle size that the timesrequired for subsequent washing and drying are unduly lengthened.

Although it is not essential to the process, it may be desirable in somecases to subject the neutralized product to a simple filtration toremove ferric hydroxide, for example, and any deleterious material thatmay have been introduced into the system with the dialkyldithiophosphoric acids. Any suitable fibrous material may be used inthis filtration step as for example a drum filled with wood shavings, abag filter, or the like.

The second step of converting the alkali metal salt to the polyvalentmetal salt composition is preferably conducted sufficiently soon afterthe neutralization step so that the formation of a gel of the alkalinemetal salt cannot occur. For example, in the case of the sodium salt,gel formation may take place at the end of about 18 hours at roomtemperature. If gel formation is permitted to occur before thedouble-decomposition reaction, the polyvalent metal salts that areproduced on double decomposition are of such small particle size thatthe times required for subsequent washing and drying of the polyvalentmetal salt are unduly lengthened.

The step of converting the alkali metal salt to the polyvalent metalsalt by double decomposition is also a fairly rapid reaction andinvolves at the most no more than about 10 to 20 minutes. This reactioninvolves very little evolution of heat and can be carried out in thesame range of temperatures as used in the neutralization step.

One important factor which affects the filterability of the finalpolyvalent metal dialkyl thiophosphate is the concentration ofelectrolytes during the double-decomposition step. This is probablyassociated with the colloidal nature of the product when it is firstformed. When Working with divalent metal chlorides, it has been foundthat for proper filterability of the final product, the concentration ofalkali metal chloride ions should be at least 2.5 moles per liter, andpreferably in the range of 2.5 to 4 moles per liter. This desiredconcentration of ions can be controlled by using the proper strength ofalkali metal hydroxide in the neutralization step and/or by using theproper concentration of polyvalent metal salt in thedouble-decomposition step. For example, in the preparation of zinc saltsof mixed C C dialkyl dithiophosphates the minimum NaOH concentrationshould be 18 to 19 weight percent; and in the double-decomposition stepthe concentration of zinc chloride should be about 50 weight percent.

Following the double-decomposition reaction, the product is washed andthen filtered.

At the end of the double-decomposition step, two phases will be formed,one of them being the polyvalent metal dialkyl thiophosphate and theother being an aqueous phase. The separation of these phases and thewaterwashing steps can be done in any convenient manner. If settling isto be conducted in tankage, it is advantageous to dilute the mixture togive a greater spread in gravity between the phases and therebyfacilitate settling and washing. Generally dilution of the system withan equal volume of water will be satisfactory for this purpose. It is tobe understood, of course, that other methods of Washing and separatingof phases can be employed, such as the use of centrifuges or othermechanical equipment. The number of water washes to which the productmust be subjected will depend largely upon the etficiency of phaseseparation; usually two washes are sufiicient. It is advisable to checkthe efliciency of the washing step by determining the alkali metalcontent of the additive, which can be done conveniently by flamephotometry. It is desirable to reduce the alkali metal content to lessthan 0.1%. The washing step is preferably conducted at a fairly hightemperature, e.g., 150 to 175 F., so as to keep the viscosity of theproduct at a low level and thus speed up phase separation.

If the system has been washed and centrifuged at an elevatedtemperature, it may not be necessary to dry the product prior tofiltration. If drying is required, one suitable method is to blow itwith air or inert gas at 180 to 200 F.

The following examples will serve to illustrate the manner in which thisinvention is to be performed, as well as the benefits derived byoperating within the defined limits of the invention. In these examplessodium Then, to the resulting solution containing the sodium salts ofthe dialkyl dithiophosphoric acids, there was added 136.5 g. of zincchloride in the form of a 50% aqueous solution. At the end of anadditional 15 mintues of agitation, the product was diluted with anequal volume of water, agitated and allowed to settle, after which theWater layer was decanted. After there such washings the product wasfiltered through diatomaceous earth and then blown with air at 180 F.until dry, which required about minutes of air blowing.

In the above preparation, the mole ratios of dialkyl dithiophosphoricacid to sodium hydroxide to zinc chloride were 1/1.1/0.5. A conversionof acid to polyvalent metal salt of 96. 8% was obtained, it beingdetermined that 9.1 g. of unreacted dialkyl dithiophosphoric acidremained.

Example 2 To establish the effect of varying the ratios of dialkyldithiophosphoric acid to sodium hydroxide to zinc chloride in thepreparation described in Example 1, a number of preparations were madeusing the procedure of Example 1 but varying the ratios as set forth inTable 11. Table I also gives information as to the quantity of unreacteddialkyl dithiophosphoric acid in each instance and the correspondingpercent conversion figure, as well as the measured pH of the product andthe mole ratios of sulfur to phosphorus, phosphorus to zinc, and sulfurto zinc in each of the products.

TABLE I Product characteristics Grams reactants Moles per mole DDPAPercent conversion pH Mole ratios DDPA NaOH ZIlOlz NaOH ZllCl2 S/P P/ZnSlZn 283 224 137 1. 10 0. 3 97 5. 80 1. 84 2. 17 3. 98 283 235 137 1. 150. 5 90 5. 90 1. 91 2. 05 3. 92 283 247 137 1. 20 0. 5 84 5. 85 l. 79 2.01 3. 61 283 258 137 1.25 0.5 81 5.90 1. 79 1. 84 3.30 283 224 143 1. 1O0. 525 99 6. 15 1. 91 2. 09 4. 01 283 235 143 1. l5 0. 525 95 6. 85 1.88 1. 99 3. 74 283 247 143 1. 20 0. 525 90 7. 0 2. 10 1. 80 3. 80 283258 143 1. 0. 525 85 7. 3 2. 08 l. 72 3. 57 283 247 137 1. 20 0. 5 85 5.85 1. 79 2. 01 3. 61 283 247 143 1. 20 0. 525 90 7. 0 2. 10 1. 80 3. 80283 247 154 1. 20 0.575 100 Washing diflicult (light mayonnaise) 283 247160 1. 20 0. 6 Completely emulsified (mayonnaise) hydroxide is employedfor preparing the alkali metal salts, since this reactant is generallythe most economic and convenient one for this purpose. It will beunderstood, however, that other alkali metal oxides or hydroxides may besimilarly employed, e.g., Na O, KOH, LiOI-l, and the like. Also, thepolyvalent metal salt is exemplified by ZnCl It is readily apparent thatother salts may be substituted in the reaction such as the chlorides,nitrates or other soluble salts of cadmium, nickel, barium, aluminum,and other metals previously mentioned.

Example 1 Mixed dialkyl dithiophosphoric acids were prepared by reactinga mixture of weight percent of primary amyl alcohols and 65 weightpercent of isobutyl alcohol with phosphorus pentasulfide in a mole ratioof alcohols to P S of 4 to 1. The reaction was conducted at about 170 F.for a period of about 4 hours until a specific gravity of about 1.05 wasattained, measured at 78 F. After the product was stripped of hydrogensulfide by the use of a stream of nitrogen, it was cooled to atemperature between 90 and 100 F. and then filtered.

A quantity of the dialkyl dithiophosphoric acids obtained as describedabove (283 g.) was neutralized by stirring into it 224 g. of sodiumhydroxide in the form of a 19.4 weight percent solution. Thisneutralization was conducted at 125 to 135 F., the temperature beingmaintained by external cooling. After all of the caustic had been added,the mixture was agitated for -15 minutes.

It will be noted from the data of Table I that although the target pH ofat least 5.8 was attained in each instance, the yields were undersirablylow when the molar ratios fell outside the range of .1 mole of dialkyldithiophosphoric acid, 1.10 to 1.15 moles of sodium hydroxide and 0.5 to0.525 mole of zinc chloride.

If excessive amounts of sodium hydroxide and zinc chloride are employed,an inversion of the colloid takes place so that instead of having adispersion of additive in the aqueous phase, the additive becomes thecontinuous phase, with the result that large amounts of salt areentrained and connot be extracted by water Washing. The productresembles mayonnaise and contains occluded water and salts that cannotbe removed by washing.

Example 3 To establish the effect of decreased concentrations of theelectrolyte on the filterability of the zinc dialkyl dithiophosphates, anumber of prepartions were made using the procedure of Example 1 withmole ratios of dialkyl dithiophosphoric acid to sodium hydroxide to zincchloride of 1/1.15/=0.525. Varying amounts of water were added to theaqueous solution of the sodium dialkyl dithiophosphate prior to the stepof double decomposition with zinc chloride. In each case the productswere washed and dried, and their filtration rates were then determinedunder standardized conditions. The concentration of electrolyte in eachcase and the filtration rates that were noted are shown in Table 11.These data show that the filtration rates drop 011 as the concentrationof electrolyte decreases from 3.7 to 1.0 moles of sodium chloride perliter and that there is a marked change when the concentration hasdropped below 3 moles per liter.

The filtration tests in which the data for Table II were obtained wereconducted as follows:

Filtration was done with a 9 cm. Buchner funnel under 20 mm. vacuum. Thefunnel was first precoated with a slurry of 25 g. of Hyfio diatomaceousearth filter aid in 250 g. of a mineral lubricating oil, the slurryhaving been heated to 180 F. After the mineral lubricating oil had runthrough the filter, the sample under test was added to the filter. Toprepare the sample for test, 2 weight percent of Hyfio filter aid wasadded to it, and it was heated to 180 R, which was the temperature offiltration. Filter rate times were measured after 25 cc. of filtratefrom the sample had been collected, thus making adequate allowance fordisplacement of essentially all of the lubricating oil from the filtercake precoat.

It is to be understood that the invention is not limited to the specificexamples herein presented, as they are merely illustrative of theinvention and the manner in which it may be practiced. The scope of theinvention is to be determined by the appended claims.

What is claimed is:

1. An improved method of preparing a polyvalent metal salt of a dialkyldithiophosphoric acid having alkyl groups in the range of from 1 to 30carbon atoms which comprises reacting the said acid with a basicmaterial selected from the group consisting of the oxides and hydroxidesof alkali metals in the proportion of 1 mole of dialkyl dithiophosphoricacid and from 1.1 to 1.15 moles of alkali metal basic material andthereafter converting the resulting alkali metal salt to a polyvalentmetal salt by a double-decomposition reaction by means of an aqueoussolution of an inorganic polyvalent metal salt, the proportion ofpolyvalent metal inorganic salt to dialkyl dithiophosphoric acid beingin the range of 0.5 to 0.550 mole of salt per mole of dialkyldithiophosphoric acid when said salt is a salt of a divalent metal, andin the range of 0.35 to 0.40 mole of inorganic salt per mole of dialkyldithiophosphoric acid when said salt is a salt of a trivalent metal.

2. Improved preparation method as defined by claim 1 wherein theconcentration of alkali metal salt ions is at least 2.5 moles per literduring the d0uble-decomposition step.

3. An improved method of preparing a zinc salt of a dialkyldithiophosphoric acid having alkyl groups of from 1 to 30 carbon atomswhich comprises the steps of neutralizing said acid with sodiumhydroxide in the proportion of 1 mole of dialkyl dithiophosphoric acidand from 1.10 to 1.15 moles of sodium hydroxide and thereaftersubjecting the resulting sodium salt to a double-decomposition reactionwith an aqueous soluton of an inorganic salt of zinc in the proportionof 1 mole of dialkyl dithiophosphoric acid to from 0.5 to 0.550 mole ofinorganic zinc salt.

4. Improved preparation method as defined by claim 3 wherein saiddouble-decomposition reaction is conducted prior to the time thatgelation has occurred following said neutralizing step.

5. Improved method as defined by claim 3 wherein said dialkyldithiophosphoric acid is derived from the reaction of a mixture ofprimary amyl alcohols and isobutyl alcohol with P 5 6. Improved methoddefined as by claim 3 wherein said neutralization step is conducted witha sodium hydroxide solution of about 18 to 20 weight percentconcentration and said double-decomposition reaction is conducted with azinc chloride solution of about 50 weight percent concentration.

7. Improved method as defined by claim 1 wherein said dialkyldithiophosphoric acid has alkyl groups of from 3 to 12 carbon atoms.

8. Improved method as defined by claim 1 wherein said polyvalent metalis a metal of Group II of the Periodic Table.

9. Improved method as defined by claim 1 wherein said polyvalent metalis zinc.

References Cited UNITED STATES PATENTS 2,476,037 7/1949 Giammaria260429] 2,488,662 11/1949 Farrington et al 260-429 3,234,250 2/ 1966Schneider et al. 260429 XR 3,290,347 12/1966 Miller 260435 1,939,95112/1933 Buchanan et al. 260987 X 2,193,965 3/1940 Hochwalt 260987 X2,391,184 12/1945 Nelson et al 260987 X 2,595,170 4/1952 Rudel et al.260987 X 2,838,557 6/1958 Verley 260987 3,014,940 12/1961 Lynch et al260987 X 3,291,817 12/ 1966 Rockett 260429.9

TOBIAS E. LEVOW, Primary Examiner.

H. M. S. SNEED, Assistant Examiner.

US. Cl. X.R.

